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Chapter 11

Chapter 11. Percent Composition and Molecular Formulas. Percent Composition. Determining the percent composition of each element in a compound H 2 O Find the molar mass of the compound Find the mass of each element in the compound (remember the subscript)

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Chapter 11

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  1. Chapter 11 Percent Composition and Molecular Formulas

  2. Percent Composition • Determining the percent composition of each element in a compound • H2O • Find the molar mass of the compound • Find the mass of each element in the compound (remember the subscript) • To find the % composition by element you must divide the mass of the element by the molar mass of the compound. Multiply by 100

  3. Formula for % composition Mass of element X 100 Mass of Compound

  4. Find the % composition by element of NaHCO3

  5. Empirical Formula • The simplest whole number ratio of the combination of elements • The empirical formula for hydrogen peroxide is HO • The molecular formula for hydrogen peroxide is H2O2 • The percent composition can be used to calculate the empirical formula. Assume that the mass of the compound is 100g if not indicated

  6. To Find the Empirical Formula • Assume the percent composition is the mass in grams if the total % for the compound = 100 • Example: SO3 • %S = 40.05 = 40.05g %O = 59.95 = 59.95g • Find the # of moles of each element in the compound. Do not round the # of moles to a whole number • Divide the smallest mole amount by itself to = 1 • Divide the remaining moles for other elements by the smallest mole amount • If a whole number still needed multiply all mole amounts by the smallest factor needed to get a whole number for all elements

  7. Determine the empirical formula for methyl acetate 48.64% carbon, 8.16% hydrogen and 43.20% oxygen

  8. Find the Empirical Formula • 36.84% N, 63.16% O

  9. Find the Empirical Formula • 60.00% C 4.44% H 35.56% O

  10. Find the Empirical Formula • 10.52 g Ni 4.38g C 5.10g N

  11. Finding the Molecular Formula • The molecular formula specifies the actual number of atoms of each element in one molecule or formula unit of the substance • To determine the molecular formula • 1. Find the n (n=# used to multiply empirical formula by) Actual molar mass of the compound = n Empirical Molar Mass of Compound 2. Multiply by the n Molecular Formula = (empirical formula)n

  12. Find the Molecular Formula of Succinic Acid • 40.68% C 5.08% H 54.24% O • Compound has a molar mass = 118.1g/mol

  13. Find the Molecular Formula • 65.45% C 5.45% H 29.09% O • Molar mass = 110.0g/mol

  14. Formula of a Hydrate • Hydrate – compound that has a specific number of water molecules bound to it • Naming a Hydrate – use a prefix to indicate the number of the hyrdate • Molar Mass of a Hydrate – is the molar mass of the compound plus the water mass • CaCl2· 2H2O • Calcium Chloride dihydrate • ℳ = 146.98g

  15. To Find the Formula of Hydrate • Find the number of moles of the compound and the water from the mass given • Find x (the moles of water) x = moles H2O moles of compound (anhydrous) • Use x as the coefficient for moles in the hydrate formula Ex: if x = 2 then 2H2O dihydrate

  16. Find the formula for CuSO4·xH2O • 2.5g blue hydrated CuSO4·xH2O is heated. After heating, 1.59 g white anhydrous CUSO4 remains. What is the formula for the hydrate? Name the hydrate

  17. Find the Formula of Hydrate • 48.8% MgSO4 51.2% H2O

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